CHEMICAL ENGINEERING cone rigidly attached at the small end to a gudgeon in the center of which jed a small supporting shaft resting in two bearings. The screen is com-ir overhung, the weight being entirely taken by the two bearings. The >r of the screen is entirely free of supporting spiders and shafts and the g spout is carried inside the screen to the small or gudgeon end. After ,rging at this point the material fed works its way through and down the in the usual trommel fashion. r heavy service such as screening boulders and large pieces of rock revolving js clothed with heavy cast plates with cored screen openings and mounted on i rollers or a combination of friction roller and gudgeon will if staunchly made reat satisfaction. Cast manganese-steel plates are the best for this type of . Contrary to common belief friction rollers consume more power in friction hafts and bearings. As it is desirable owing to great wear and tear not to my spiders and shafts in devices of this kind the friction wheel mounting is Isory but only at the receiving or feed end of the screen. At the other end the can be supported by a gudgeon and shaft which affords a simple way of driving helps to reduce the power consumed in friction.1 e common diameters for trommels are 36, 42 and 48 in, and the corresponding ;r of revolutions per minute fixed by practice is 20, 18 and 17. otion of a Particle in a Trommel.—The theory of the motion of the particles ierfectly or incompletely set forth in the treatises on "Ore Dressing" but completely in works of this kind than in other publications which have id upon screening. In the trommel the material after being fed forms a or less deep bank or mass occupying the whole length of the screen for a n of the circumference near the bottom but to the side of the center line. »ank is on the side to which an arrow showing the direction of rotation point, the rotation arrow being placed at- the bottom of the screen. As ommel rotates the bank rises and falls and at the same time the particles ce to a degree dependent upon the slope of the trommel. For a single le the path described would be a series of curved and connected saw teeth. single particle of some material introduced into a cylindrical trommel in rota-tarts from the lowest position at the bottom and rises with the motion of the on an arc at right angles to its long axis and partakes entirely of the motion trommel and is in contact with it up to the point where the tangential com-b of its weight makes a line which coincides with the angle of friction, and then omewhat higher point due to the increased normally acting pressure caused trifugal force, which of course varies with the speed of rotation of the trommel, this point it will rise still higher by momentum. It then falls down toward ttom. The rate of rotation must be very much below that which would cause rticle to cling to the screen surface and be carried completely around with it in itions. It is evident that the more the bank slides up and down over the screen j the greater the opportunities for the undersize grains to pass through the res, on the other hand the higher the bank is carried at each swing the farther the particles will fall in their sliding down paths. Richards has estimated particle will be carried completely around a 30-in. trommel when it has a rate ,tion of 64 times per minute. Similar figures for a 36-in. and a 48-in. trommel pectively 58.4 and 50.6 r.p.m. The practical figures for rate of rotation which excellent design of revolving grizzly is shown on p. 275. "The Theory and Practice of Ore 5," McGraw-Hill, 1915. Description of the details of trommels and their driving, housing, etc., ound in this work and Richard's "Ore Dressing."